At present, techniques such as using shoot tips and axillary buds of potato and using cutting sections of young plantlets for propagating plantable virus-free seedlings and further producing virus-free potato minitubers have been adopted. There are a number of publications dealing with the production of virus-free potato minitubers.
Chinese Patent Application No. 87106041.8 disclosed a mass production of viroid-free and virus-free potato propagation material, which included separating tissue cells from potato stem-tips, culturing them in nutrient medium, and then inducing them to take roots or microtubers. The following is the detailed procedure: culturing the virus-free test-tube plantlets in a liquid without nutrient at 22.degree. C. and 3000 lux in culture flasks numbered by plants, after about three weeks, replacing the liquid in the flasks by a nutrient solution to accelerate tuberization(which consists of phenyladenine 10 mg/l, coumarin 10 mg/l, sucrose 8% and some inorganic nitrogen source etc.), culturing them at 17.degree. C. and 1000 lux for another 10-12 weeks. 40-60 potato microtubers with 0.3-0.6 cm in diameter are thus developed. The procedure for minitubers is as follows: planting the microtubers above in a mixture of peat and pearlite containing 0.5 Kg/m.sup.2 of urea at the density of 400 plants per square meter, watering the plants under natural temperature and illumination, after eight weeks adding a plant growth regulator (a chlorine-choline-chloride) to inhibit the growth of leaves and, at the same time, stopping watering 3-4 minituber stocks with 0.5-2.0 cm in diameter are thus obtained. The disadvantages of this method are long growth period, namely 10-12 weeks for microtubers and 85-90 days for minitubers, and, accordingly, high cost of production. In addition, the whole process is conducted in a laboratory and a lot of organic compounds are consumed. Therefore, it is not suitable for practical mass production and has high cost.
Chinese Patent Application No. 89106287.4 disclosed a method of propagating and planting potatoes, which included cutting test-tube plantlets into sections with an axillary bud on each section, treating the stem section under the leaf with growth and rooting-accelerating compounds and then transplanting the sections into nutrient medium within rolled plastic film, (the nutrient medium is made of tiny peat from marshland with adding fertilizer and trace or common elements such as (NH.sub.4).sub.3 PO.sub.4, NH.sub.4 NO.sub.3, K.sub.2 SO.sub.4, MgSO.sub.4, Fe.sub.2 (SO.sub.4).sub.3, CuSO.sub.4, ZnSO.sub.4, HBO.sub.3, NH.sub.4 MO.sub.4, rock powder etc. and its pH is adjusted to 5.0-5.5), spraying water onto the peat and covering the film rolls with polyethylene film, keeping the plants so growing for another 20-25 days. 4-6 sections can be cut from each plant and be transplanted into film rolls when further propagation is wanted. Plantlets with well developed roots can be obtained directly from these sections growing in film rolls and be transplanted into open field to produce potato tubers. 40-50 tubers with the average weight of 25-29 g can then be produced from each plant. The disadvantages of this method are huge propagation of seedlings and large spaces needed for them, inconvenience of management, high production cost and inconvenient long distance transport of lots of seedlings as well.
Chinese Patent Application No. 90101337.4 disclosed a mass production method of seed potatoes (potato microtubers), which was an improved method of mass production of pathogen-free seed potatoes (potato microtubers) by tissue culture technique of plant, producing test-tube plantlets in flat-bottomed culture disks with the disadvantages of long growth cycle, very high production cost and smaller microtubers.